test_elemwise4 passed; modified setting of stride in alloc_contiguous to set stride 0 for dimensions of size 1

added a fct to return the instance of the default mode. It check that it exist and raise an informative error if it don't exist.

In function_module there is a check in infer_reuse_pattern, which is wrong. I talked about it with James, he did a new version of infer_reuse_pattern and assumed that the new version should always return at least what the old version was returning. It turns out this is not the case and that the new version is correct.